If you don't already have enough DLC for these achievements I recommend doing all of the other DLC achievements first. Several of them require specific songs or songs from certain lists, so you'll end up getting much closer to 20 songs by the time you finish them. New copies of the various track packs (AC/DC, Country, TP2, Metal, etc) can also all be exported for free from any non-used copies. Any of them would give you more than enough songs for this achievement.

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